Method for producing piezoelectric film actuator, and composite structure having piezoelectric layer

ABSTRACT

A method for producing a piezoelectric film actuator is provided. This method includes the steps of preparing an intermediate transfer member having a porous layer formed thereon, with a vibrating plate and a piezoelectric layer being provided on the porous layer; bonding the vibrating plate to a nozzle substrate to form a composite structure; and separating the intermediate transfer member from the composite structure at the porous layer to transfer the vibrating plate and the piezoelectric layer to the nozzle substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods for producing piezoelectricfilm actuators for use in, for example, liquid-ejecting heads.

2. Description of the Related Art

Known liquid-ejecting heads are disclosed in, for example, the followingpublications.

(1) Japanese Patent Laid-Open No. 2002-134806

According to this publication, a vibrating plate composite andpiezoelectric films are provided to cavity parts by forming thepiezoelectric films on an intermediate transfer member, bonding thevibrating plate composite to the piezoelectric films, and separating theintermediate transfer member to transfer the piezoelectric films to thevibrating plate composite.

(2) Japanese Patent Laid-Open No. 2002-234156

This publication discloses a piezoelectric element composite including asingle-crystal or polycrystalline vibrating plate sandwiched betweenoxide layers and uniaxial-crystal or single-crystal piezoelectric films.

A liquid-ejecting head is provided by sequentially forming a SiO₂ film,a YSZ film, Pt films, and PZT films, which function as piezoelectricfilms, on an SOI substrate, partially etching the Si substrate to formpressure chambers, and bonding an intermediate Si substrate and anorifice plate that constitute parts of the pressure chambers.

(3) Japanese Patent No. 2976479

This publication discloses a liquid-ejecting head including a nozzlesubstrate having nozzles, a silicon substrate having cavitiescommunicating with the nozzles and thin film parts corresponding to thecavities, and pressure generators formed on the thin film partsintegrally without any bonding step.

(4) Japanese Patent Laid-Open No. 10-286953

According to this publication, electrodes, lead-based dielectric layers,electrodes, and a vibrating plate are sequentially formed on a MgOsubstrate. After resin or glass for forming pressure chambers areprovided on the vibrating plate, the MgO substrate is partially orcompletely removed. The pressure chambers are formed by, for example,patterning.

First Problem: According to Japanese Patent Laid-Open No. 2002-134806,after the piezoelectric films are transferred to the vibrating platecomposite, a wiring step is required to provide, for example, electricalpaths to the piezoelectric films. This step involves limitations such asthe need for avoiding the breakage of ink cavities. Such limitationsmake it difficult to support flexibility in, for example, finerprocessing and heat processes.

Second Problem: The liquid-ejecting head according to Japanese PatentLaid-Open No. 2002-234156 has the following two problems.

(1) Use of an SOI Wafer leads to high production cost because the waferis more expensive than general silicon wafers.

(2) If the selective etching of the Si wafer for forming pressurechambers is performed by alkali etching to ensure etching selectivity tothe buried silicon oxide, the openings of the pressure chambers becomelarger than the areas thereof adjacent to the vibrating plate. This isdisadvantageous in arranging the pressure chambers at higher density.

Third Problem: Japanese Patent No. 2976479 is disadvantageous in termsof the thickness control of the vibrating plate. As the amount of eachdroplet ejected is reduced to improve the resolution and gradation ofink jet printers, the number of ink dots per unit area on printing paperis increased. Accordingly, the responsiveness of the vibrating platemust be enhanced to maintain printing speed by, for example, reducingthe thickness of the vibrating plate. The reduction in the thickness ofthe silicon vibrating plate, however, may cause a problem in thicknesscontrol because selective etching based on the difference in impurityconcentration has low selectivity.

Fourth Problem: According to Japanese Patent Laid-Open No. 10-286953,the use of the MgO substrate makes it difficult to form peripheralcircuitry, such as a drive circuit, using single-crystal silicon as anactive layer. This is disadvantageous in terms of the finer processingof ink jet heads, which is accompanied by higher density. Because inkjet heads with higher densities require higher-speed drive circuits, itis desired to form peripheral circuitry on single-crystal silicon, whichis a high-mobility material. If a Si substrate is used, however, thevibrating plate cannot be made of Si. This causes difficulty inachieving a high-quality single-crystal vibrating plate with goodsensitivity.

SUMMARY OF THE INVENTION

In light of the above problems, the present invention provides apiezoelectric film actuator that includes a vibrating plate with finelycontrolled thickness, may be produced without the use of an SOI wafer,and is insusceptible to heat processes, and also provides a method forproducing the piezoelectric film actuator.

A method for producing a piezoelectric film actuator according to thepresent invention includes the steps of preparing a first substratehaving a porous layer formed thereon, with a vibrating plate and apiezoelectric layer being provided on the porous layer; bonding thevibrating plate to a second substrate to form a composite structure; andseparating the first substrate from the composite structure at theporous layer to transfer the vibrating plate and the piezoelectric layerto the second substrate.

In yet another aspect, the present invention relates to a compositestructure including a first substrate on which a porous layer isdisposed; a piezoelectric layer; a vibrating plate disposed between theporous layer and the piezoelectric layer; and a second substrate towhich the vibrating plate is bonded, wherein the porous layer isdisposed between the first substrate and the vibrating plate.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings, in which like reference characters designate the sameor similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

FIGS. 1A to 1C are diagrams showing the main steps of a method forproducing a piezoelectric film actuator according to the presentinvention.

FIGS. 2A to 2C are diagrams showing the steps of preparing a nozzlesubstrate according to the present invention.

FIGS. 3A to 3D are diagrams showing the steps of producing apiezoelectric film actuator with a piezoelectric substrate and thenozzle substrate according to the present invention.

FIGS. 4A and 4B are diagrams of a piezoelectric substrate according toan example of the present invention after the formation of resist films.

FIGS. 5A and 5B are diagrams of the piezoelectric substrate in the stepfollowing the step in FIGS. 4A and 4B.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will now be described in detailwith reference to the drawings. FIGS. 1A to 1C are diagrams showing themain steps of a method for producing a piezoelectric film actuator(hereinafter also simply referred to as an actuator) according to thepresent invention. As shown in FIGS. 1A to 1C, the method for producinga piezoelectric film actuator according to the present inventionincludes the main steps of forming a porous silicon layer on a firstsubstrate made of silicon (hereinafter also referred to as anintermediate transfer member); forming a vibrating plate on the porouslayer; and forming a piezoelectric layer on the vibrating plate. Thismethod further includes the steps of bonding the vibrating plate to asecond substrate to form a composite-structure; and separating theintermediate transfer member from the composite structure at the porouslayer to transfer the vibrating plate and the piezoelectric layer to thesecond substrate. The individual steps according to the presentinvention will be specifically described below.

1. Formation of Vibrating Plate, Piezoelectric Film, and Electrode (1)Formation of Porous Silicon Layer (FIG. 1A)

Referring to FIG. 1A, a porous Si layer 11 is formed on one of the mainsurfaces, which are polished, of a single-crystal Si substrate 10 havinga thickness of 625 μm. The porous layer 11 is intended to facilitate theremoval of the intermediate transfer member after the vibrating plateand the piezoelectric film are bonded to cavities. The porous layer 11is formed by, for example, anodizing, in which current is applied withthe main surface of the single-crystal Si substrate 10 as an anode in anaqueous HF solution.

The thickness of the porous layer 11 is defined by, for example,controlling the time for anodizing. The porous layer 11 may have abilayer structure including an exposed first layer with lower porosityand an underlying second layer with higher porosity than the firstlayer. The bilayer structure allows stress to concentrate at the part ofthe second layer in the vicinity of the interface between the first andsecond layers so that they can be separated selectively at that part.

The thickness' of the porous layer 11 is not particularly limited. Theporous layer 11 may have a thickness of 0.01 to 200 μm, particularly 0.5to 10 μm; excessive thickness causes large wafer warpage which mayinterfere with the process. If the porous layer 11 has the bilayerstructure, the first layer generally has a thickness of 20 μm or less,particularly 10 μm or less, and the second layer generally has athickness of 10 μm or less, particularly 5 μm or less. The porosity ofthe first layer is generally 60% or less, particularly 30% or less; highporosity degrades the quality (e.g., crystal defect density and surfaceroughness) of an epitaxial layer.

The porosity may be determined according to the change in wafer weightbefore and after the anodizing and the thickness of the porous layer 11.

(2) Formation of Vibrating Plate (FIG. 1B)

After the anodizing, a vibrating plate 12 is formed on the porous layer11. The vibrating plate 12 may be made of Si, though the material usedis not limited to Si. The vibrating plate 12 may be formed by, forexample, thermal CVD, plasma-enhanced CVD, MBE, or liquid-phase epitaxy.

The vibrating plate 12 is made of a material with a Young's modulus of50 GPa or more. Examples of such a material include stainless steel, Ti,zirconia, Si, Cu, SiO₂, glass, and Cr. The vibrating plate 12 may haveeither a monolayer structure or a multilayer structure. For themultilayer structure, the total Young's modulus must be 50 GPa or more.The vibrating plate 12 may have a thickness of 0.5 to 20 μm,particularly 1 to 10 μm. The material for the vibrating plate 12 may bedoped with a slight amount of metal such as Y or B.

In particular, Si may be used for the vibrating plate 12 since a drivecircuit, for example, can be formed in a normal Si process.

(3) Formation of Pressure Generator (FIG. 1C)

A piezoelectric layer made of PZT and accompanying electrode layers areformed on the vibrating plate 12, which is a nonporous single-crystalfilm, by, for example, the following process.

A common electrode layer 21, a piezoelectric layer 20, and an individualelectrode layer 22 are formed on the vibrating plate 12 by sputtering orion plating. The common electrode layer 21 is made of, for example, Pt,Cr, or Ni and has a thickness of 1 μm. The piezoelectric layer 20 ismade of PZT and has a thickness of 10 μm. The individual electrode layer22 is made of, for example, Pt, Cr, or Ni. A patterned resist film isformed on the individual electrode layer 22. The common electrode layer21, the individual electrode layer 22, and the piezoelectric layer 20are etched using the resist film as a mask by ion etching or reactiveion etching to form common electrodes 21′, individual electrodes 22′,and piezoelectric layers 20′. Additionally, vibrating parts and wiringparts are formed at the same time.

2. Formation of Nozzle and Production of Ink Jet Head (4) Formation ofNozzle Substrate (FIGS. 2A to 2C)

Next, a process for preparing a nozzle substrate A2 and its structurewill be described with reference to FIGS. 2A to 2C. The nozzle substrateA2 may be made of any material that can form nozzles. Examples of thematerial for the nozzle substrate A2 include glass, resin, andsingle-crystal Si. In particular, a single-crystal Si substrate may beused because it has the same thermal expansion coefficient as thepiezoelectric substrate A1 and good resistance to aging. Nozzles areformed by, for example, the following process.

Referring to FIGS. 2A to 2C, a single-crystal Si substrate 60 with bothsurfaces thereof polished that has a thickness of 100 μm is prepared.SiO₂ films 61 having a thickness of 0.1 μm are formed on both surfacesof the single-crystal Si substrate 60 by thermal oxidation. A resistlayer 63 is formed on the surface of one SiO₂ film 61 other than squareregions 64 having the same dimensions as the openings of cavities to beformed on the nozzle substrate A2 such that the square regions 64 havesides in the [110] directions, while another resist layer 63 is formedon the overall surface of the other SiO₂ film 61 (FIG. 2A).

The portions of the SiO₂ film 61 in the square regions 64 are removed byetching, and then the resist layers 63 are removed. Subsequently, thesingle-crystal Si substrate 60 is anisotropically etched with an aqueoussolution of pyrocatechol and ethylenediamine (FIG. 2B), and the SiO₂films 61 are removed. The resultant nozzles 70 have outlets 71 smallerthan the cavity openings (FIG. 2C).

The nozzle substrate A2 may also be formed by laminating two substratesmade of different materials or the same material. In this case, a cavityportion and a nozzle portion can be separately formed. Examples of thematerial used include glass, resin, and single-crystal Si. Inparticular, a single-crystal Si substrate may be used because it has thesame thermal expansion coefficient as the piezoelectric substrate A1 andgood resistance to aging.

(5) Bonding of Piezoelectric Substrate and Nozzle Substrate (FIG. 3A)

The piezoelectric substrate A1 and the nozzle substrate A2 thus producedare bonded to form a composite structure such that the piezoelectriclayers 20′ and the nozzle outlets 71 are directed to the ejection side.These substrates A1 and A2 may be bonded by, for example, anodicbonding, active metal brazing, or the use of adhesive or covalentbonding.

Even an adhesive with low heat resistance may be used since the drivecircuit, for example, has already been formed.

(6) Separation of Intermediate Transfer Member from Vibrating Plate andPiezoelectric Element (FIG. 3B)

Referring to FIG. 3B, the intermediate transfer member (single-crystalSi substrate) 10 is separated from the composite structure, which is anactuator including the vibrating plate 12 and the piezoelectric layers20′ bonded thereto, at the porous layer 11 to transfer the vibratingplate 12 and the piezoelectric layers 20′ to the nozzle substrate A2.The intermediate transfer member 10 may be separated by, for example,destroying the porous layer 11 mechanically or with water jets, or byrapid heating through laser irradiation.

Japanese Patent No. 2877800, to the assignee of the present application,discloses a method for separating a composite member, such as bondedsubstrates, with a fluid.

(7) Removal of Porous Layer (FIG. 3C)

If the porous layer 11 is not exposed after the separation of theintermediate transfer member 10, the porous layer 11 is exposed by, forexample, lapping, grinding, polishing, or etching. This exposure step isnot required if the porous layer 11 has already been exposed.

Subsequently, the porous layer 11 is etched with, for example, ahydrofluoric acid solution. The etchant used may be a hydrofluoric acidsolution particularly if the porous layer 11 is oxidized in advance. Theetchant used, however, is not limited to a hydrofluoric acid solution;an aqueous alkali solution, for example, may be used if no oxide isformed on the pore walls of the porous layer 11 or if oxide is removedin advance.

(8) Formation of Penetrating Electrode (FIG. 3D)

After the removal of the porous layer 11, if necessary, electrical pathsto the pressure generators are provided from above since the pressuregenerators are disposed in the nozzle substrate A2. Penetratingelectrodes 13 may be readily provided if the vibrating plate 12 is madeof a thin film having a thickness of about 0.2 μm.

The penetrating electrodes 13 may be formed by any method with a metalsuch as Ni, Sn, W, Fe, Cu, Au, or Pt.

Example

Next, the present invention will be described with reference to anexample below, though the invention is not limited to the example.

FIGS. 3A to 3D, 4A and 4B, and 5A and 5B are schematic diagrams showingthe steps of a method for producing a liquid-ejecting head including apiezoelectric film actuator according to a specific example of thepresent invention.

(1) Formation of Porous Silicon

In the step of forming a porous layer on an intermediate transfermember, a Si crystal substrate having a thickness of 625 μm was used asthe intermediate transfer member. A porous Si layer was provided byforming an exposed first layer under the conditions of a current densityof 8 mA/cm² and a processing time of 5 to 11 minutes and then forming anunderlying second layer under the conditions of a current density of 23to 33 mA/cm² and a processing time of 80 seconds to 2 minutes in a mixedsolution of an aqueous HF solution (HF concentration: 49% by weight) andethanol in a ratio of 2:1 by volume. The first layer had a porosity ofabout 20% and a thickness of about 6 μm while the second layer had aporosity of about 50% and a thickness of about 3 μm.

(2) Formation of Vibrating Plate

A single-crystal Si layer was allowed to grow on the porous Si layer asa vibrating plate by CVD. In the initial growth stage, a non-porous filmwas allowed to grow at a low rate by supplying a slight amount, namely20 nm/min or less, of starting material to seal pores on the surface ofthe porous Si layer before a single-crystal Si layer having a thicknessof 10 μm was formed at a growth rate of 1 μm/min.

(3) Formation of Pressure Generator

FIGS. 4A and 4B are diagrams of the piezoelectric substrate A1 after theformation of resist films. FIG. 4A is a top view of the piezoelectricsubstrate A1, and FIG. 4B is a sectional view taken along line 4B-4B′ inFIG. 4A. A common electrode layer 21, a piezoelectric layer 20, and anindividual electrode layer 22 were formed on the vibrating plate 12 bysputtering or ion plating. The common electrode layer 21 was made of,for example, Pt, Cr, or Ni and had a thickness of 1 μm. Thepiezoelectric layer 20 was made of PZT and had a thickness of 10 μm. Theindividual electrode layer 22 was made of, for example, Pt, Cr, or Ni.Resist films 23 were formed on the individual electrode layer 22. Theseresist films 23 had dimensions of 425 μm by 800 μm, and the sidesthereof were aligned in the directions indicated by arrows X-X′ and Y-Y′in the drawing.

FIGS. 5A and 5B are diagrams of the piezoelectric substrate A1 in thesubsequent step. FIG. 5A is a top view of the piezoelectric substrateA1, and FIG. 5B is a sectional view taken along line 5B-5B′ in FIG. 5A.The common electrode layer 21, the individual electrode layer 22, andthe piezoelectric layer 20, shown in FIGS. 4A and 4B, were etched by ionetching or reactive ion etching using the pattern of the resist films 23to form common electrodes 21′, individual electrodes 22′, andpiezoelectric layers 20′ (pressure generators 26).

(4) Formation of Nozzle Substrate

Referring to FIGS. 2A to 2C, a single-crystal Si substrate 60 with bothsurfaces thereof polished that had a thickness of 100 μm was prepared.SiO₂ films 61 having a thickness of 0.1 μm were formed on both surfacesof the single-crystal Si substrate 60 by thermal oxidation. A resistlayer 63 was formed on the surface of one SiO₂ film 61 other than squareregions 64 having the same dimensions as the openings of cavities to beformed on the nozzle substrate A2 such that the square regions 64 hadsides in the [110] directions, while another resist layer 63 was formedon the overall surface of the other SiO₂ film 61 (FIG. 2A). The portionsof the SiO₂ film 61 in the square regions 64 were removed by etching,and then the resist layers 63 were removed. Subsequently, thesingle-crystal Si substrate 60 was anisotropically etched with anaqueous solution of pyrocatechol and ethylenediamine (FIG. 2B), and theSiO₂ films 61 were removed. The resultant nozzles 70 had outlets 71smaller than the cavity openings (FIG. 2C).

(5) Bonding

In the subsequent step of bonding the vibrating plate and the pressuregenerators disposed on the intermediate transfer member to the nozzlesubstrate, the piezoelectric substrate A1 and the nozzle substrate A2were brought into contact such that the piezoelectric layers 20′ and thenozzle outlets 71 were directed to the ejection side. These substratesA1 and A2 were subjected to anodic bonding at 400° C. by applying avoltage of 1,000 V across the piezoelectric substrate A1, at a minuspotential, and the nozzle substrate A2, at a plus potential.

(6) Separation

The intermediate transfer member could be readily separated at theporous layer by exposing the porous layer to a water jet with a pressureof 15×10⁴ kPa.

(7) Formation of Electrode

After etching, a SiO₂ mask was formed on the vibrating plate, which wasetched to a depth of 10 μm from the wafer surface by dry etching to formopenings having dimensions of 10 μm by 10 μm directly over the pressuregenerators. These openings were filled with Cu by plating, and the Cuwas mechanically polished to form flat surfaces. The SiO₂ films wereremoved to provide electrodes.

The liquid-ejecting head thus produced could stably eject liquid at arate of 11 m/s with a drive voltage of 20 V and a frequency of 15 kHz.

According to the present invention, a vibrating plate may be depositedon an intermediate transfer member to facilitate the control ofthickness of the vibrating plate and provide more uniform thickness.This has the advantage that the thickness of the vibrating plate can befreely adjusted according to the performance of pressure generators. Inaddition, normal semiconductor equipment may be used.

The steps of forming the vibrating plate and the pressure generators maybe separated from the steps of forming a nozzle substrate havingcavities. This has the advantage that the material for the vibratingplate, as an actuator, may be widely selected, and the material for thenozzle substrate may be freely selected.

In addition, the present invention avoids limitations associated withhandling and heat processes in a wiring step for forming, for example, adrive circuit because the wiring step is finished before the bonding ofthe nozzle substrate.

Furthermore, the intermediate transfer member may be readily separatedat a porous layer mechanically or by laser irradiation without damagingthe vibrating plate. This allows the recycling of the intermediatetransfer member.

Accordingly, the present invention can provide a piezoelectric filmactuator capable of producing large displacement at a low drive voltage,responding quickly, producing large power, and achieving higher area anddensity. In addition, the present invention can provide a long,high-density liquid-ejecting head capable of producing largedisplacement at a low drive voltage and providing quick response andhigh reliability.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications, equivalent structures and functions.

This application claims the benefit of Japanese Application No.2004-258366 filed Sep. 6, 2004, which is hereby incorporated byreference herein in its entirety.

1. A method for producing a piezoelectric film actuator, comprising thesteps of: preparing a first substrate having a porous layer formedthereon, with a vibrating plate and a piezoelectric layer being providedon the porous layer; bonding the vibrating plate to a second substrateto form a composite structure; and separating the first substrate fromthe composite structure at the porous layer to transfer the vibratingplate and the piezoelectric layer to the second substrate.
 2. The methodfor producing a piezoelectric film actuator according to claim 1,wherein the first substrate comprises silicon. 3-10. (canceled)
 11. Themethod for producing a piezoelectric film actuator according to claim 1,wherein the piezoelectric layer is formed by etching using a resist filmas a mask.
 12. The method for producing a piezoelectric film actuatoraccording to claim 1, wherein the vibrating plate has a multilayerstructure. 13-20. (canceled)